Yes, there is decay for Rank Points, but don’t worry - decay only occurs at rank Platinum I and above. You must be inactive for 15 days for Point Decay to begin. After your 15 days, 5 Rank Points are lost per day of inactivity in the Ranked queue, up to a maximum amount of 200 Rank Points lost.
Elo, which is used to matchmake you with similarly skilled opponents, does not decay.